The etiology of osteoporosis, a skeletal disease characterized by low bone mass and increased fracture risk, remains incompletely understood. Recent studies by our group and others have implicated dietary protein-induced metabolic acid loading and consequent net renal acid excretion (NRAE) as causes of increased urine calcium and bone resorption in young, adult women. Whether (a) similar phenomena are exaggerated in elderly women, (b) buffer administration ameliorates such effects, and (c) protein increases bone resorption through TNF-related cytokines, or osteoprotegerin (OPG), or receptor activator for nuclear factor (KB)-ligand (RANKL) remain unknown. Our study is designed to answer these questions. An ambulatory, ethnically diverse population of 80 elderly, female volunteers will receive low-protein diets (0.8 g/kg bodyweight), followed by high-protein diets (1.6 g/kg bodyweight), followed then by high protein diets plus concommitant oral potassium bicarbonate buffer. Blood and urine collections will be performed during each treatment period to determine the effects of dietary protein and buffer administration on NRAE, urine calcium, serum indices of bone resorption and formation, and serum OPG and RANKL.